This invention relates to an electrographic process for producing, an electrostatic charge image corresponding to an image to be recorded on a record sheet having a dielectric material layer, by modulating a flow of corona ions by through an electrostatic latent image formed on a photosensitive screen.
Various kinds of such processes have heretofore been proposed. One of these processes which can freely and precisely control a picture image makes use of a photosensitive screen composed of four layers consisting of an insulating layer, two electric conductive layers formed on diametrically opposite sides of the insulating layer and a photosensitive layer formed on one of the two electric conductive layers. This process comprises a first step of uniformly charging the above mentioned photosensitive layer; a second step of illuminating the photosensitive layer with a light image to form an electrostatic latent image thereon; and a third step comprising locating a record sheet having a dielectric material layer in opposition to the photosensitive layer of the photosensitive screen. Thereafter a high electric voltage is applied across a grounded field electrode provided in the rear of the record sheet having the dielectric material layer and the electric conductive layer of the photosensitive screen. Next a bias electric voltage is applied across the two electric conductive layers of the photosensitive screen and then a flow of ions is directed from a corona discharge device arranged at the side of and spaced apart from the electric conductive layer of the photosensitive screen, through the photosensitive screen and toward the record sheet having the dielectric material layer whereby, an electrostatic charge image which corresponds to the electrostatic latent image formed on the photosensitive layer of the photosensitive screen is produced on the record sheet having the dielectric material layer.
Such conventional process has the advantage that the picture image can be controlled by adjusting the bias electric voltage applied across the two electric conductive layers of the photosensitive screen composed of the four layers and that a plurality of copies can be obtained by repeating the above mentioned third step after the electrostatic latent image has been formed on the photosensitive screen.
However, experimental tests have indicated that in such a process, if a plurality of copies are to be obtained, a picture image concentration becomes changed as a function of the number of copies. It is considered that such change of the picture image concentration is caused by dark attenuation properties of a photoconductive body and an undesirous curved flow of ions directed from the corona discharge device toward the record sheet. As a result, the above mentioned process has the disadvantage that copies having good quality picture image are limited in number and that a very large number of copies could not be obtained.